Only limit etcd memory on small hosts (#1860)

Also disable oom killer on etcd

roles/etcd/defaults/main.yml

@@ -17,7 +17,8 @@ etcd_election_timeout: "5000"
 etcd_metrics: "basic"
 
 # Limits
-etcd_memory_limit: 512M
+# Limit memory only if <4GB memory on host. 0=unlimited
+etcd_memory_limit: "{% if ansible_memtotal_mb < 4096 %}512M{% else %}0{% endif %}"
 
 # Uncomment to set CPU share for etcd
 # etcd_cpu_limit: 300m